Antenna



A. S. DUNBAR ANTENNA Filed March 26, 1946 MR mm EN VU m D s N E L L A.

m L W Y E N w W A Patented Oct. 30, 1951 I ANTENNA Allen S. Dunbar, Watertown, Mass., assignor, by mesne assignments, to the United States of America as represented by the Secretary of War Application March 26, 1946, Serial No. 657,142

1 This invention relates to antennas for high frequency communication systems and more par-' ticularly to a beacon antenna suitable for use in high speed aircraft and projectiles.

The installation of radio or radar controlled beacon systems in high speed aircraft and in certain projectiles, such as are known as robot bombs, present a need for a beacon antenna having the following characteristics; small size for minimum drag, unit gain, a substantially uniform' azimuth radiation pattern sufficiently broad in elevation to permit maneuvering of the aircraft or projectile, and broad band characteristics. Beacon antennas heretofore proposed although having certain of the desired characteristics have,

been found to be much too bulkyfor use on aircraft, projectiles or other similar vehicles designed for high speed. It has been found that a vertically polarized sleeve radiating antenna according to this invention possesses all of the desired characteristics particularly having the advantage of small size with consequent small drag or wind resistance.

Accordingly, it is an object of the present invention to provide an antenna having the abovementioned characteristics.

It is another object of the invention to provide a beacon antenna suitable for use on high speed aircraft, radar controlled projectiles or similar vehicles and which is characterized by being of simple construction and having very small extension beyond the skin line of the vehicle thereof.

For a better understanding of the invention together with other and further objects thereof, reference is had to the following description, taken in connection with the accompanying drawing:

The single figure is partly a side elevation view and partly a broken away longitudinal sectional view of the antenna according to a preferred embodiment of the invention.

In the drawing, I designates the surface or skin of an aircraft, projectile or other similar vehicle of the antenna herein described. Extending from or projecting through and beyond the surface It is the antenna II according to the present invention. Antenna II generally comprises a coaxial conductor having a center conductor I2 and an outer concentric conductor I3. The inner ends of conductors I2 and I3 are adapted to be connected or coupled or may be an extension of an inclosed transmission line such as a coaxial conductor line which feeds energy from a source (not shown) withinthe vehicle to antenna II. The outer end ll of outer conductor 1Claim. (c1. 250-:33)

I3 extends as a circular sleeve projection fora short distance a: beyond the skin or surface Ill; The inner or central conductor I2 has its outer end portion I5 extending outwardly beyond the end of sleeve I4 with the end thereof being spaced a distance y beyond the skin l0.

The distance r is not more than one-eighth of the wavelength of the radiated energy andis usually considerably less preferably being one twenty-fifth of the wavelength to provide better impedance matching and 'performance. Distance 1/ representing the effective length of the projecting end portion I5 of the inner conductor I2, which portion may be termed a radiator, is preferably about one-fourth wavelength I although it may be somewhat less than a quarter Wavelength when the radiator I5 is made relatively thick in cross-sectional dimensions for broad banding purposes.

As a support for inner conductor I2 including radiating portion I5, a washer or bead I6 of a suitable dielectric material may be inserted between inner conductor I2 and sleeve M or as shown may be seated against the end of sleeve I4, bead I6 having a bore I! through which inner conductor I2 is snugly received. A hollow cap I8 of a suitable dielectric material is preferably fitted over the radiating portion I5 to protect theantenna II from the effect of variations in weather and/0r pressure. The annular base I9 of cap I8 may be secured to the surface In in any suitable manner. Preferably, however, a flat circular base plate 20 which may serve as a reflecting surface is secured to the surface It) and a circular or annular flange member 2| is secured to plate 20, the flange 22 of member 2| being adapted to overlap the annular base portion I9 of cap I8. For ease of production and assembly outer conductor I3, sleeve portion I4. plate 20 and. member 2| may be cast or otherwise formed as an integral piece. The inner surface of cap I 8 is preferably provided with a shoulder 23 adapted to seat against bead I6 for maintaining the-bead I6 and inner conductor l2 in position.

The cross-sectional shape of the radiating portion l and of the cap l8 may be either circular as shown or it may be elliptical or streamlined in shape, the elliptical or streamlined shape preferably being utilized when the antenna II is used on very high speed vehicles to provide minimum drag. For example, the radiating portion [5 may be an ellipse in cross-section with the majorand minor axes having a ratio of the order of six to one. The radiation patterns produced by the antenna I I having circular, elliptical or streamlined radiators and caps have beenfound practically identical and uniform in azimuth, assuming the antenna axis to be disposed in a vertical manner, although the non-circular radiators are slightly less broad band than the circular radiators.

ture, a co-axial transmission line mounted Certain aircraft and vehicles offer difficult 7 installation problems in that external attachments such as auxiliary fuel tanks or bombs or parts of the aircraft such as engine nacelles, horizontal or vertical stabilizers, etc., tend to obstruct the view of the antenna in azimuth especially at small angles'of roll or pitch. Consequently, such external attachments must be taken into consideration when mounting the antenna. To overcome the possibleadvantages, the antenna instead of being mounted on the skin or surface of the vehicle itself may be mounted onan airfoil or other suitable projection from the skin surface of the aircraft proper. For example, such an airfoil may be suspended slightly below the aircraft thus permitting a certain amount of roll and pitch without obstructing the antenna and producing by proper shaping of the airfoil a pattern in the transverse plane which is broad enough to permit the aircraft to roll or bank through an angle of or more.

i It has been found that an antenna constructed as described herein has all of the desired characteristics from considerations of aerodynamics and electrical performance which have not heretofore been achieved by any antenna heretofore known or proposed.

While there has been described what is at through said aperture, said line having an outer conductor securedto said base plate and projecting beyond said plate for a distance of substantially one twenty-fifth of a wavelength at the operating freqency, said line having an inner conductor mounted coaxially in said outer conductor and projecting beyond said base plate for a distance of substantially one quarter of a wavelength, the projecting portions of said inner and outer conductors being adapted to radiate and receive waves of electromagnetic energy, and a hollow cap of dielectric material mounted over the projecting portions of said inner and outer conductors and shaped to conform to the cross"? sectional shape of said projecting portions, said cap being spaced from said projection of said inner conductor and having a base portion adja cent saidv projection of said outer conductor and secured to said base plate. ALLEN S. DUNBAR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,235,506 Schelkunoff Mar. 18, 1941 2,239,724 Lindenblad Apr. 29, 1941 2,313,046 Bruce Mar. 9, 1943. 2,368,663 Kandoian Feb. 6, I945 

